Outline

Introduction to Casting

Casting process

Material is melted

Heated to proper temperature

Treated to modify its chemical makeup

Molten material is poured into a mold

Solidifies

Advantages of Casting

Complex shapes
Parts can have hollow sections or cavities
Very large parts
Intricate shaping of metals that are difficult to machine
Different mold materials can be used
Sand, metal, or ceramics
Different pouring methods

Disadvantages

Porosity
Dimensional variations due to shrinkage
Trapped impurities, solid and gaseous
High quality surface finish is difficult/impossible
Toughness not comparable with forging

Basic Casting Processes

1. Mold cavity is produced having the desired shape and size of the part

Takes shrinkage into account

Single-use or permanent mold

Patterns

2. Melting process

Provides molten material at the proper temperature

3. Pouring technique

Molten metal is poured into the mold at a proper rate to ensure that erosion and or defects are minimized

Gravity pouring

Forcing

4. Solidification process

Controlled solidification allows the product to have desired properties

Mold should be designed so that shrinkage is controlled

5. Mold removal

The casting is removed from the mold

Single-use molds are broken away from the casting

Permanent molds must be designed so that removal does not damage the part

Solidification Process

Molten material is allowed to solidify into the final shape
Casting defects occur during solidification
Gas porosity
Shrinkage
Two stages of solidification
Nucleation
Growth
Solidifying the Material
Removing heat
Absorbed by the mold (conduction)
Radiated away (radiation)
Air may be circulated around the casting (convection)
Drying
Hardening
Chemical reaction (RIM)

Nucleation
Stable particles form from the liquid metal
Occurs when there is a net release of energy from the liquid
Undercooling is the difference between the melting point and the temperature at which nucleation occurs
Each nucleation event produces a grain
Nucleation is promoted (more grains) for enhanced material properties
Inoculation or grain refinement is the process of introducing solid particles to promote nucleation
Grain Growth
Occurs as the heat of fusion is extracted from the liquid
Direction, rate, and type of growth can be controlled
Controlled by the way in which heat is removed
Rates of nucleation and growth control the size and shape of the crystals
Faster cooling rates generally produce finer grain sizesCooling Curves
Useful for studying the solidification process
Cooling rate is the slop of the cooling curve
Solidification can occur over a range of temperatures in alloys
Beginning and end of solidification are indicated by changes in slope

Prediction of Solidification Time: Chvorinov’s Rule

Ability to remove heat from a casting is related to the surface area through which the heat is removed and the environment that it is rejecting heat to.Chvorinov’s Rule

Where t is the solidification time, V is the volume of the casting, A is the surface area of the casting that contacts the mold, n is a constant, and B is the mold constant. The mold constant B depends on the properties of the metal, such as density, heat capacity, heat of fusion and superheat, and the mold, such as initial temperature, density, thermal conductivity, heat capacity and wall Thickness. According to Askeland, the constant n is usually 2, however Degarmo claims its between 1.5 and 2.It is most useful in determining if a riser will solidify before the casting, because if the riser solidifies first then it is worthless.

Molten Metal Problems
Chemical reactions can occur between molten metal and its surroundings
Reactions can lead to defects in the final castings
Metal oxides may form when molten metal reacts with oxygen
Dross or slag is the material that can be carried with the molten metal during pouring and filling of the mold
Affects the surface finish, machinability, and mechanical properties
Gas porosity
Gas that is not rejected from the liquid metal may be trapped upon solidification
Several techniques to prevent gas porosity
Prevent the gas from initially dissolving in the liquid
Melting can be done in a vacuum
Melting can be done in environments with low-solubility gases
Minimize turbulence
Vacuum degassing removes the gas from the liquid before it is poured into the castings
Gas flushing- passing inert gases or reactive gases through the liquid metal

Fluidity and Pouring Temperature
Metal should flow into all regions of the mold cavity and then solidify
Fluidity is the ability of a metal to flow and fill a mold
Affects the minimum section thickness, maximum length of a thin section, fineness of detail, ability to fill mold extremities
Dependent on the composition, freezing temperature, freezing range, and surface tension
Most important controlling factor is pouring temperature

The Role of the Gating System
Gating system delivers the molten metal to the mold cavity
Controls the speed of liquid metal flow and the cooling that occurs during flow
Rapid rates of filling can produce erosion of the mold cavity
Can result in the entrapment of mold material in the final casting
Cross sectional areas of the channels regulate flows

Gating Systems
Proper design minimizes turbulence
Turbulence promotes absorption of gases, oxidation, and mold erosion
Choke- smallest cross-sectional area in the gating system
Runner extensions and wells- used to catch and trap the first metal to enter the mold and prevent it from entering the mold cavity
Filters- used to trap foreign material

Solidification Shrinkage
Most metals undergo noticeable volumetric contraction when cooled
Three principle stages of shrinkage:
Shrinkage of liquid as it cools from the solidification temperature
Solidification shrinkage as the liquid turns into solid
Solid metal contraction as the solidified metal cools to room temperature

Solidification Shrinkage
Amount of liquid metal contraction depends on the coefficient of thermal contraction and the amount of superheat
As the liquid metal solidifies, the atomic structure normally becomes more efficient and significant amounts of shrinkage can occur
Cavities and voids can be prevented by designing the casting to have directional solidification
Hot tears can occur when there is significant tensile stress on the surface of the casting material

Risers and Riser Design
Risers are reservoirs of liquid metal that feed extra metal to the mold to compensate for shrinkage
Risers are designed to conserve metal
Located so that directional solidification occurs from the extremities of the mold toward the riser
Should feed directly to the thickest regions of the casting
Blind riser- contained entirely within the mold cavity
Live riser- receive the last hot metal that enters the mold
Riser must be separated from the casting upon completion so the connection area must be as small as possible
Riser Aids
Riser’s performance may be enhanced by speeding the solidification of the casting (chills) or slowing down the solidification (sleeves or toppings)
External chills
Masses of high-heat capacity material placed in the mold
Absorb heat and accelerate cooling in specific regions
Internal chills
Pieces of metal that are placed in the mold cavity and promote rapid solidification
Ultimately become part of the cast part

Patterns

Two basic categories for casting processes
Expendable mold processes
Permanent mold processes
Patterns are made from wood, metal, foam, or plastic
Dimensional modification are incorporated into the design (allowances)
Shrinkage allowance is the most important
Pattern must be slightly larger than the desired part

Pattern Removal
Parting lines are the preferred method
Damage can be done to the casting at corners or parting surfaces if tapers or draft angles are not used in the pattern
Factors that influence the needed draft
Size and shape of pattern
Depth of mold cavity
Method used to withdraw pattern
Pattern material
Mold material
Molding procedure

Design Considerations

Design Considerations in Castings

Location and orientation of the partingline is important to castings
Parting line can affect:
Number of cores
Method of supporting cores
Use of effective and economical gating
Weight of the final casting
Final dimensional accuracy
Ease of molding

It is often desirable to minimize the use of cores
Controlling the solidification process is important to producing quality castings
Thicker or heavier sections will cool more slowly, so chills should be used
If section thicknesses must change, gradual is better
If they are not gradual, stress concentration points can be created
Fillets or radii can be used to minimize stress concentration points
Risers can also be used

Drafts

Section Thicknesses

Design Modifications
Hot spots are areas of the material that cool more slowly than other locations
Function of part geometry
Localized shrinkage may occur

Design Modifications
Parts that have ribs may experience cracking due to contraction
Ribs may be staggered to prevent cracking
An excess of material may appear around the parting line
The parting line may be moved to improve appearance
Thin-walled castings should be designed with extra caution to prevent cracking

Design Modifications

Casting Designs
May be aided by computer simulation
Mold filling may be modeled with fluid flow software
Heat transfer models can predict solidification